Glowworm micron 60ff?

Hi, I have a Glowworm Micron 60FF boiler, which keeps resetting. Reset light on solid. It happens once or twice a day. In between the resets, the boiler works fine.
The reset light indicates overtemp, and I've monitored the temp on the feed pipe, and it normally heats up to about 65, before turning off and cooling, and back up to 65. Every so often it goes up to 85, followed by the reset. The board, thermistor, pump and overtemp stat have all been replaced, but the fault remains. As far as I can see it has to be the thermistor or board. The old and new thermistors test ok.
Any ideas? Old board was 2000227135, new board is 2000801991.